I see you are still on 4.3. Are you rooted? Have you done anything to prevent the latest 3 updates from being installed?
Do SMS and MMS work properly with mobile data on and wifi calling off? I'm talking about having Wifi turned on but "wifi calling" off and not doing anything to disable mobile data.
Are you using the stock messaging app? MMS over Wifi Calling often do not work in third party apps.
While MMS over Wifi Calling should work if you are properly registered with the T-Mobile network, if will usuallly be slower than over mobile data. But you should not be getting any error message if you are registered with the network. The screen shot I had wanted was the status screen that shows the network and service state and roaming.
Lastly, if you are properly registered on the TMO network when on Wifi Calling and you are using the stock messaging app, there could be a bug in your older version of software. I never ran 4.3 on this phone so I can't personally say if there was any problem in this area with that version.